The field of the present invention is radial inflow turbine expanders.
Radial inflow turbine expanders which employ variable primary nozzles have a reasonably wide range of flow. Such turbine expanders, or turboexpanders as they are often referred to, include nozzle blades which are pivotally mounted parallel to the axis of the turbine wheel and arranged in an annular inlet about the inlet to the turbine wheel. These blades may be caused to vary in orientation so as to increase or decrease the nozzle area between blades. In this way, the turbine may be adjusted to accommodate a range of flows with maximum practical efficiency. A recent patent illustrating one system contemplated for use with the present invention is U.S. Pat. No. 4,300,869, for Method & Apparatus for Controlling Clamping Forces in Fluid Flow Control Assemblies to Swearingen, the disclosure of which is incorporated herein by reference. See also, U.S. Pat. Nos. 3,232,581 and 3,495,921, also incorporated herein by reference.
Associated with such variable inlet nozzle turbines are secondary nozzles located at the discharge of the turbine wheel and defined by the blades of the wheel. These secondary nozzles are necessarily of fixed cross-sectional area and serve to jet the discharge from the turbine wheel backward as it leaves the wheel relative to the motion of the wheel. In doing so, the flow thus discharged may be arranged to leave the turbine wheel through the discharge with no angular momentum. In this way, the energy otherwise lost in spinning flow discharged from the turbine is avoided in favor of the realization of additional useful power to the turbine.
In such radial inflow turbines, reduced flow is accommodated by adjusting the inflow nozzles. The flow which is discharged from the turbine wheel tends to be thrown outwardly by centrifugal force such that the inner portion of the flow nearest to the axis of the turbine wheel at the discharge will be substantially diminished while flow near the periphery of the discharge will still better approximate the flow at optimum flow rates. As a result, the secondary nozzles still perform reasonably well to reduce angular momentum in the discharge. Naturally, the unavoidable fixed losses in the turbine must be prorated against a smaller flow. Efficiency is correspondingly diminished. This diminution in efficiency is generally unavoidable.
Flows larger than the design flow or optimum flow of said device are generally accommodated by the opening to a greater extent of the primary nozzles. The secondary nozzles are fixed and must simply accommodate more flow through the same nozzle area. In order to do so, the flow velocity must be increased. This induces a swirl in the discharge which naturally usurps energy from the system. Additionally, the secondary nozzles require additional differential pressure to establish the higher flow of velocity. Because of this additional pressure energy requirement, less energy is available for the primary nozzles. As a result, the primary stream is introduced tangentially into the turbine wheel at lower than optimum velocities. Further losses are experienced because of the velocity mismatch between the inlet flow from the primary nozzles and the peripheral speed of the turbine wheel. The flow impacts upon the turbine wheel because of the mismatch, resulting in reduced efficiency.
Because of the natural accommodation of below optimum flow rates in such radial inflow turbines, the major efficiency losses are understood to occur at flow rates above the optimum flow rate of the device. The major losses at higher than optimum flow rates are understood to be impact loss at the turbine wheel inlet, the loss due to angular momentum of the gas at the discharge and the passing of excessive flow at elevated pressures through the fixed secondary nozzles. In spite of such losses, many systems employing turboexpanders experience variations in flow rate both below and above the optimum.